I know this is an older topic but thought I would chime in on it after recently reviewing the aviation ratings regs I have copies of. This type of Balloon Observer with the O on the face of the balloon is not a 1930's pre-WWII wing. This badge was created in 1940 when they added a separate Balloon Pilot rating (which took over the badge without the O) and also added the Senior Balloon Pilot rating. So balloon wings without the O might be pre-WWII Balloon Observer wings, but those with the O would not be.
This is per War Department Circular No. 31, Dated March 23, 1940 which had changes to AR 600-35 for Aviation Badges from the prior revision dated December 31, 1926. The eight new badges were also illustrated in War Department drawing 4-4-1 dated April 19, 1940.